Add code symbols into outline

[vezwork]

Addresses an issue mentioned by @juliasilge: #167 (comment)

Adds code symbols under code cells in the outline.

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[vezwork]

I attempted to add tests until I remembered that our test setup does not have actual LSPs for Python or R in it, so we can't really test this. Well... now that I write that, I realize I could probably mock one out. That seems a little contrived, but maybe still worth it.

[juliasilge]

This PR:

• fixes Feature Request: In qmd file outline view, in source mode, include R functions (that are in R chunks) #880
• fixes Feature Request: Add Outline Support for Named Code Sections in R Code Chunks #918

[juliasilge]

I seem to be getting two of everything, at least in Positron:

[vezwork]

I was able to reproduce duplicated language symbols in code cells in Positron with inline outputs enabled. Looking into what is happening.

[vezwork]

@juliasilge In qmds in Positron with Positron › Quarto › Inline Output: Enabled enabled, the following command (from getCodeCellSymbols in this PR) is returning duplicated symbols:

await commands.executeCommand<DocumentSymbol[] | SymbolInformation[]>(
  "vscode.executeDocumentSymbolProvider",
  uri
);

I checked the surrounding context, including the vdoc content. Everything seems to be in order, nothing strange otherwise. When I disabled Positron › Quarto › Inline Output: Enabled and re-open the document, I no longer experience the duplicated symbols issue. It appears to me that the "vscode.executeDocumentSymbolProvider" command does not function as expected when inline outputs are enabled.

[vezwork]

@seeM mocked out fake LSPs in #980. Maybe I can copy that approach for tests in this PR.